This invention relates to the production of coal in situ using pairs of vertical wells linked together through the coal seam. More particularly the invention teaches the use of a movable oxidizer injection point so that the underground reaction zone can be positioned away from the oxidizer injection well.
This invention extends the teachings of my U.S. Pat. No. 4,185,692, which is incorporated herein by reference. In reference patent methods were taught of accomplishing underground linkage using one or more blind hole burns.
One of the significant problems encountered in full scale field tests of producing coal in situ is that of the high temperatures occurring around the injection well. Generally the injection well is bottomed in the lower portion of the coal seam, with the linkage to the production well also positioned at or near the bottom of the seam. Oxidizer is injected at this point to sustain gasification. Initially only a portion of the oxidizer is consumed in the linkage, with the balance used to consume coal around the injection well. In the early stages of production a generally cylindrical cavity is burned the full depth of the seam from the bottom to the top of the seam before any appreciable cavity growth occurs in the linkage channel. Radius of this cylinder is generally in the order of 15 feet and the temperature of the burn is high enough to cause substantial damage to the lower part of the casing--often burning away the casing thus exposed. The oxidizer injection point that began at the bottom of the seam is thus changed at the top of the seam, generally creating a flame override situation with respect to the linkage channel.
Thus it is apparent that it is highly desirable to have an oxidizer injection point in the linkage channel sufficiently removed from the injection well to prevent burning of a cavity around the injection well casing. It is an object of the present invention to teach the use of a movable oxidizer injection point.
Some work in this general area has been accomplished by Lawrence Livermore National Laboratory (LLNL) in field tests recently conducted in the State of Washington. These tests involved drilling a horizontal well at the bottom of a 20' seam at the high wall of a strip mine. This well was completed as an injection well which was intercepted at its extremity by a vertical well through the overburden. A reaction zone was established using the vertical well as the production well. Coal was gasified for a period of several days, injection was terminated and the injection tubing was shortened. A second reaction zone was then established apart from the original burned out cavity. LLNL has identified this procedure as the controlled retractable injection point. In this procedure during the first burn,casing damage is sustained by the production well in addition to damage to the original end of the injection tubing. Since the injection tubing extremity is considered expendable, no particular damage occurs to the injection well.